1. Field of the Invention
The present invention relates to wavelength division communications, and more particularly to a method and an apparatus for generating a visible light signal which corrects the energy differences among wavelengths when parallel transmission is implemented by using the multiple wavelengths in a visible light communication system.
2. Description of the Related Art
Recently, as Light Emitting Diodes (LEDs) have been improved in luminous efficiency, LEDs are more commonly used not only in a special illumination market, such as handheld devices, displays, automobiles, traffic lights, advertising boards, etc., but also in a general illumination market, such as fluorescent lamps, incandescent electric lamps, etc. Also, as interest in optical wireless technology complementary with RF technology has increased because of a number of different reasons (e.g., exhaustion of the frequencies in a Radio Frequency (RF) band, possibility of interference of RF communication among wireless communications, an increase of the security requirement for communications, the advent of a very high-speed ubiquitous communication environment of fourth generation mobile communication (4G) wireless technology, etc.,) studies are being carried out on optical wireless communications using visible light LEDs in many enterprises and research institutes, etc.
The visible light communications for transmitting information by using visible light have merit, such as a wide use band and the ability to be freely used without being subject to regulation. Also, the visible light communications have merit in that the reception range of information can be accurately sensed because a spot where light reaches or a direction in which the light moves can be seen. Accordingly, the visible light communications have a reliability in an aspect of security, and also have merit such as the ability to be driven with low electric power in another aspect of power consumption.
Luminous elements for the visible light communications have made rapid progress lately, but are not yet able to turn on/off at high speed. To cite an example, in a case of a white LED using phosphor, its manufacturing cost is cheap but its modulation speed is no more than about 10 Mega-bits per second (Mbps). In order to overcome this limitation, studies are proceeding on a scheme in which visible light having information is generated by using multiple LEDs for generating three primary colors, including Red, Green, and Blue (RGB), and the generated colored lights are mixed to make white light. A scheme of transmitting signals in parallel by using the multiple LEDs for generating the three primary colors has merit in that high-speed transmission can be implemented, but in a case where the balance among respective energy distributions of wavelengths are upset, there appears a problem such that generated light can have a color tone other than white. If the generated light corresponds to light having any color tone other than the white light, this becomes a serious disadvantage in the visible light communications in which a transmitter serves as a lighting device. A description will be made of a conventional apparatus for wavelength division parallel visible light communications, which transmits signals in parallel by using the multiple LEDs.
FIG. 1 is a block configuration diagram illustrating an example of a conventional transceiver for wavelength division parallel visible light communications. With reference to FIG. 1, a transmitter 101 for visible light communications includes multiple encoders 105-1 thru 105-n, multiple modulators 111-1 thru 111-n, a light generator 120, and a controller 103. Herein, the multiple encoders 105 are configured in parallel, and perform channel coding on data to be transmitted. The multiple modulators 111 are configured in parallel, and modulate respective channel-coded data from the multiple encoders 105. The light generator 120 transmits signals modulated by the multiple modulators 111 as visible signals. The controller 103 controls each configuration element of the transmitter 101 for visible light communications.
A receiver 102 for visible light communications includes a light sensor 125 multiple demodulators 112-1 thru 112-n, multiple decoders 106-1 thru 106-n, and a controller 104. Herein, the light sensor 125 receives visible light signals. The multiple demodulators 112 are configured in order to demodulate the visible light signals received by the light sensor. The multiple decoders 106 receive respective signals demodulated by the multiple demodulators 112, perform channel decoding on the respective received signals in order to restore the respective received signals to their original states. The controller 104 controls configuration elements of the receiver 102 for visible light communications. In the apparatus for wavelength division parallel visible light communications, paths independently operate by each path wherein path information is to be transmitted has been determined.
FIG. 2 is a flowchart illustrating transmission/receive (Tx/Rx) operations of wavelength division parallel visible light communications in general. Referring to FIG. 2, the transceiver for visible light communications begins to operate. In step 210, the transmitter for visible light communications transmits a visible light signal, and in step 220, the receiver for visible light receives the visible light signal from the transmitter for visible light communications. Thereafter, in step 230, it is determined whether the Tx/Rx operations are completed. If it is determined that the Tx/Rx operations are completed, in step 240, the Tx/Rx operations of visible light communications are completed. If not, the procedure returns back to step 210, and the Tx/Rx operations are repeatedly performed until the Tx/Rx operations are completed.
In the meantime, a color balance refers to a state where final mixing light has white color as energy distributions in the specific ratio are achieved by each of the wavelengths of Red, Green, Blue visible lights. A mixture of light is determined according to the energy ratio among the three primary colors. Namely, the energy distribution rate according to wavelength of light determines a color tone of the light. Specially, the white light has electric power existing over all wavelength bands. The relation between energy by wavelengths and a color tone of the light can be found with reference to a chromaticity diagram.
Since parallel transmission can be achieved if different information is transmitted by each wavelength, high-speed luminous elements are not required. The receiver filters a received light signal through an optical filter, and can extract and recover only a signal having a desired wavelength. Because different information is transmitted by each wavelength if the scheme of parallel transmission using the wavelength division is viewed from the aspect of color balance, an energy balance may not be kept among multiple wavelengths, and light generated from the loss of the energy balance cannot be white light. If the generated light corresponds to light other than white light, this becomes a serious problem in the visible light communications in which the transmitter serves as the lighting device at the same time.
Also, because the visible light communications using wavelength division correspond to a system in which free space propagation is performed, the visible light communications are affected by a phenomenon where wireless channels are time-varying. This phenomenon implies that the communications are implemented in an environment different from a communication system using optical fibers. There is diversity technique among techniques for coping with a time-varying channel in a general Radio Frequency (RF) communication system. The diversity refers to technique for retransmitting data to be transmitted over several times by allocating other channel resources (e.g., time, frequency, codes, space) to the data to be transmitted so as to guarantee more accurate transmission/receive in a communication system.
Furthermore, an iterative encoding technique is one of the techniques used for providing a stable transmission. The iterative encoding technique corresponds to the simplest channel encoding technique, and refers to a method for repeating transmission of the same signal on the assumption that a transceiver has already perceived a pattern.
However, in a case where the prior diversity technique and the iterative encoding technique are applied to a visible light communication system, as the imbalance of electric power among wavelengths causes a difference of electric power among the wavelengths to become larger, the more difficult the generation of white light becomes.